Tanks for liquefied gases

ABSTRACT

1,177,231. Liquefied gas containers. G. J. H. TREPAUD. 27 Jan., 1967 [28 Jan., 1966; 10 June, 1966; 12 Jan., 1967], No. 4071/67. Heading F4P. In the event of an undue external temperature rise, liquefied gas e.g. butane stored under pressure at or below a level 16 in an inner tank 1, Fig. 1, is caused to rise up the annular space 7 between tank 1 and a surrounding tank 5 thereby cooling the latter and the so-vaporized butane is freed of entrained liquid particles by passage through a conventional separator 13, 14, 15 and is then passed through a duct 12 containing a bursting disc 23 to an open-air burner system (not shown). The bottom portions of tanks 1, 5 are connected by ducts 8, 8&lt;SP&gt;1&lt;/SP&gt; having normally closed valves 10, 10&lt;SP&gt;1&lt;/SP&gt; which open at a predetermined temperature to allow access of liquid butane to annular space 7 and also through apertures 20, 21 in hollow tank-supporting legs 2. Liquid particles separated by an annular baffle 14 and a filter mesh 15 are returned through a funnel 13 and a vertical duct 3 to the lower portion of the outer tank 5. The latter is coated externally by an asbestos layer 6. In a modification the upper and lower portions of inner tank 1, Fig. 4, are open to the outer tank 5 and liquid forced up space 7 is vaporized,. freed from liquids by baffles 45, 13, 14 and is relieved through valves 70, 70&lt;SP&gt;1&lt;/SP&gt; to the open-air burner. In a further modification, when the upper and lower portions of the inner tank 1, Fig. 5,. are open to the tank 5, the inner tank is formed of separate segments 1a, 1b, Fig. 7, secured along their edges to spaced rectangular sectional ducts 46, 46&lt;SP&gt;1&lt;/SP&gt; which extend in vertical meridian planes and are welded to the outer tank 5. The duets are open at their lower ends 47, 47&lt;SP&gt;1&lt;/SP&gt; and are connected at their upper ends 48, 48&lt;SP&gt;1&lt;/SP&gt; to atmosphere through a fuzible disc 50, Fig. 6 and a next duct 49 and also to the inner tank through a non-return valve 52.

Feb. 4, 1969 G, J. H. TREPAUD TANKS FOR LIQUEFIED GASES sheen; l' om.

Filed Jan. 26, 1967 Feb. 4., 1969 G. J. H. T REPAUD TANKS PoR LIQUEFIEDGASES Fuse aan. 26. 1967 Sh'eet g of 4 Feb 4i 1969 G. J. H. TREPAUD3,425,234

TANKS vFOR LIQUEFIE'D GASES Filed Jan. 26, 1967 Sheet 3 of 4 Feb. 4,1969 Sheet Filed Jan. 26, 1967 United States Patent O 3,425,234 TANKSFOR LIQUEFIED GASES Georges Jean Henri Trepaud, 44 Rue la Boetie, Paris,France Filed Jan. 26, 1967, Ser. No. 611,979 Claims priority,application France, Jan. 28, 1966, 47,651; June 10, 1966, 64,962; Jan.12, 1967, 90,845

U.S. Cl. 62-45 25 Claims Int. Cl. F17c 1/12, 7/00 ABSTRACT F THEDISCLOSURE A tank for a liquefied gas, of the type comprising an innerand an outer wall enclosing a narrow, annular space, Means are providedto connect to each other the respective lower ends of this annular space.and of the inside of the inner wall, at least when the temperatureoutside of thetank increases dangerously; thereby the liquefied gasrising upwardly through the annular space is at least partly vaporizedby absorbing the heat transmitted from the outside of the tank. Meansare further provided, near to the upper end of the annular space, toseparate the vaporized gas from the still liquefied gas, to exhaust thevaporized gas in the atmosphere, and to return the still liquefied gasdown to the lower end of the annular space.

Background of the invention This invention relates t0 a tank forliquefied gas, notably for low-molecular weight hydrocarbons, whichincorporates ia safety device for protecting the tank against the risksof explosion in case of abnormal increase of the external or surroundingtemperature.

It is known that liquefied gases, notably hydrocarbons that are gaseousunder ordinary temperature land pressure conditions, are frequentlystored in the liquid state and under pressure in closed and sealed tanksin which the liquefied gas remains in equilibrium, at room temperature,with its satunating vapour, during a relatively long storage periodpreceding its industrial or domestic use.

Recent accidents, as murderous as spectacular, proved that a tank ofthis type can burst when submitted to the intense heat released by anearby fire of which the probability is all the less negligible as thesetanks are generally assembled at a relatively high density rate on thestorage sites of oil refineries.

T-he tank according to this invention is of the type broadly set forthhereinabove but it is protected against any risk of explosion, even incase of violent fire, and characterised in that it comprises a doublewall providing an annular chamber in which, in case of abnormal increasein the external or surrounding temperature, notably in case of fire,liquefied gas rises from the tank and absorbs by evaporating, at leastone fraction of the heat transferred through the external wall, thusavoiding the bursting of the tank, and also in that known separatingmeans are provided lat the upper end of said annular chamber forseparating the thus evaporated gas from the still liquefied gas,together with a system of open-air burners for venting the vaporized andseparated gas.

In case of increase in the external temperature the liquefied gascontained in the tank according to the present invention will thus risefrom the bottom to the top of the annular chamber formed between thedouble wall of the tank, and the fiux of external heat received by thetank and transferred through its external wall will cause the liquefiedgas rising in said annular chamber to boil; as a continuous circulation-of liquefied gas is maintained between the two ends of the annularchamber the vaporized fractions of said gas tare vented t0 theatmosphere through said external or open-air burners, preferably at icea relatively great distance from the seat of the fire, so as to beburned thereat without supplying fresh fuel to said fire. In thefollowing disclosure it will be proved that instead of rising thetemperature and pressure prevailing 1n the tank according to thisinvention will subsequently decrease gradually and that in any case thewhole of the tank content can be evacuated gradually in the mannerexplained hereinabove, until the external fire has been put undercontrol or has ceased, without allowing any risk of explosion to appearat any time.

In a first form of embodiment of the t'ank according to this inventionthe thickness of the inner wall is adapted to the maximum permissibleinternal pressure of the liquefied gas contained therein, and the outerwall is considerably thinner, the annular chamber formed between thesetwo walls, which is normally closed, being filled with an inert gas suchas nitrogen under a relatively low overpressure, said tank furthercomprising means for causing the lower end of said annular chamber tocommunicate with the tank bottom when the external temperature and/orthe internal pressure exceed a dangerous value, and also a return linefor establishing in this case a continuous circulation of boilingliquefied gas from the lower end to the upper end of said annularchamber.

When a fire breaks out in the vicinity of a tank of known type simplyequipped with a safety valve, the liquefied gas stored therein begins toboil tumultuously and unless the level of liquefied gas stored in thistank is extremely low the inlets of the safety valves are flooded withliquefied gas which will vaporize only if allowed to expand as itemerges therefrom, that is, outside the tank, in the open-air burnersystem communicating with the atmosphere. The heat transmitted from theoutside through the wall of the known tank will thus partake completelyin the temperature increment yand therefore in the pressure increment ofthe liquefied gas contained in the tank; under these conditions thesafety valves provided therein are inadequate for preventing theexplosion of this known type of tank, which is inevitable when theinternal pressure exceeds the test or rated pressure. Moreover, thesesafety valves are subjected to a veritable bombardment by liquid massesthrown in all directions within the tank of known type, said safetyvalves being therefore more or less damaged by this bombardment, so thatif they open it is most likely that they will be unable to besubsequently reclosed in a fluid-tight manner, even if the innerpressure in the known tank eventually drops, for example if the fire hasbeen put out; as a result, the safety valves permit the escape of thewhole of the gas contained in the tank of known type from the verymoment an increase, even lof temporary nature, of the internal pressurehas caused these valves to open. Under these conditions it is clear thateven an otherwise small fire is sufficient to cause the loss of theconsiderable mass of gas contained in a tank of the known type.

Another form of embodiment of the tank according to this invention isfree of the inconveniences mentioned hereinabove although it comprisessafety valves opening into the surrounding atmosphere as in the knowntank type.

This second form of embodiment of this present invention ischaracterised in that the external wall of the tank has a thicknessconsistent with the -maximum permissible internal pressure, that theinternal wall is considerably thinner and has its upper and lower endswidely open t0 cause said annular chamber of the double-,wall tankstructure to communicate with the inner space of the tank, and thatsafety valves gaged to open at predetermined internal pressures areinterposed between the aforesaid separator means and the open-air burnersystem.

As these separator means are disposed at the upper end of the annularchamber of the double-wall structure of the tank according to thisinvention, before or u-pstream of the safety valves, the gaseous massescaping through these valves will actually carry along but microscopicdroplets of liquid fuel which are not capable of submerging or damagingsaid safety valves, or preventing the yfiuid-tight reclosing thereof, asobserved in known tank types, in the absence of said separator means, bythe mixture of gas and liquid escaping tumultuously from the upper endof said annular chamber.

Nearly the whole of the heat transmitted from the outside through thetank wall is thus absorbed by the evaporation of the liquefied gas inthe annular chamber, so that the temperature and therefore the pressureof the liquefied gas still contained in the inner wall are kept atsubstantially constant values, thus precluding any risk of explosion.

This increased Safety is due to the fact that the safety or exhaustfvalves equipping this second form of embodiment of the tank accordingto this invention are fed only with nearly dry vaporized gas, when theyare open, as a consequence of the point action of the annularvaporization chamber and of said separator means, as contrasted with thesafety valves of known tanks which are practically fiooded with boilingliquefied gas.

According to an advantageous modification of the above disclosed secondform of embodiment lof the tank of this invention, a plurality of groupsof safety valves are provided, the respective valves of each group beinggaged to open at stepped internal pressure valves and having thereforelikewise stepped cross-sectional passage areas, whereby, for instancethe valves of only one group which lhave a relatively smallcross-sectional passage area will open in case of intense sun radiation,the valves of all the groups opening in case of fire.

In the case of the second form of embodiment of the tank according tothis invention, when, in case of fire, the partial vaporization of theliquefied gas contained in the tank has strongly reduced the leveltherein (for example to less than 3 feet in the case of a spherical tankof a diameter approximating 40 feet) only practically dry and evenoverheated vaporized gas circulates in the upper portion of the annularchamber of the double-wall structure, this vaporized gas being therefore11nable to absorb the heat transmitted from the external fire throughthe external 'wall of the tank. In other words, the temperature of theexternal vvall of the tank may rise, notably towards the upper end ofthe annular chamber, as the level of liquefied gas decreases in lchetank, to a value whereat the strength of the material constituting saidexternal wall (as a rule steel) is not sufficient to withstand thecorresponding internal pressure. This may be the cause of a partialbreaking down of the outer wall of the tank and give rise to seriousrisks in case a certain mass of liquefied gas is still present in thebottom of said tank, the sudden vaporization of this residual gas beingmost likely under these conditions to produce a violent explosion.

'Ilhis drawback may be avoided -by resorting to the two improvements setforth hereinafter and applicable to the second form of embodiment of thetank according to this invention, independently of, or possibly incombination with, each other:

According to the first improvement a set of substantially vertical ductsor tubes are disposed against the inner face of the outer wall, saidducts or tubes being disposed at spaced intervals along the horizontalsection or circumference of the annular chamber, the lower end of eachd-uct leading into the tank, near the lower end of said annular chamber,and the upper end of eaoh duct opens into an atmosphere exhaust memberconnected for example to the open-air burner system, the opening of thismember being controlled by the temperature attained by the outer wall incase of fire, when the partial vaporization of the liquefied gascontained in the tank has caused the level therein to drop to such adegree that only vaporized and overheated gas circulates in the -upperportion of the annular chamber.

With this first improvement when the temperature of the outer wall hasreached a critical value for the material constituting this wall, incase of fire, the upper end of each duct is caused to communicatethrough the relevant exhaust member with the atmosphere, whereby thesmall mass of gas still in the liquid state which remains in the bottomof the tank is rapidly forced by the pressure prevailing within the tankinto the lower ends of said ducts where the liquefied gas rises and isfinally vented t0 the atmosphere for example through said open-airburner system; under these conditions the tank is drained completely andany risks of explosion as a consequence of the breaking of the outerwall are thus definitely preeluded, since only gas is contained therein.

According to a second improvement applicable in case the tank weresupported above the ground by legs or the like, its double-wallstructure comprises a cylindrical downward skirt-like extension ofsmaller transverse dimensions, said skirt-like extension being likewisedouble-walled so that when the tank proper has been nearly exhausted bya fire having vaporized the liquefied gas therein, a small volume of gasstill in the liquid state remains in this hollow extension. As thissmall residual mass would lvery rapidly evaporate if the external firecontinued, this second improvement will enable the tank to be drainedout completely, thus definitely avoiding any risks of explosion causedby a partial breaking of the lupper portion of its outer wall.

Reference will now be made to the accompanying drawings illustratingdiagrammatically by way of example various forms of embodiment of thetank according to this invention. nI the drawings:

FIGUR-E 1 is a vertical section, taken along a diametral plane, of afirst form of embodiment of the spherical tank according to thisinvention;

FIGURE 2 is a fragmentary section showing on a larger scale of a detail,the section being taken upon the line II II of FIGURE l;

FIGURE 3 is another fragmentary section showing 0n a larger scale detailof a leg, according to a modified embodiment of the tank structureillustrated in FIGURE 1; v FIGURE 4 is another vertical section takenalong a diametral plane of a second form of embodiment of the sphericaltank according to this invention;

FIGURES 5 and 8 show respectively in vertical section two improvementsconcerning the tank illustrated in FIGURE 4;

FIGURE 6 shows on a larger scale the detail A of FIGURE 5, and

FIGURE 7 is a fragmentary section taken upon the line VII- VII of FIGURE5.

The spherical tank illustrated diagrammatically in FIGURE 1 is intendedmore particularly for the aboveground storage of liquefied hydrocarbons,such as propane, ethane, butane, etc. It comprises essentially a closedand fluid-tight external wall 5, consisting of a hallow steel sphere forexample of a diameter of 40 feet, its wall thickness, for example in thecase of liquid propane storlage, being of the order of 13716 to 1%6".This sphere 1 is supported by sturdy tubular metal legs, of which arelatively great number, for example eight, are disposed along its outerperiphery, although only two legs are visible in the figure anddesignated by the reference numerals 2. According to this invention, thesphere 1 has mounted therein, coaxially to its vertical diameter, areturn tubular duct 3 of a diameter ranging for example from 20 to 24",for a purpose to be explained presently; the two ends of this duct arewelded lin a fluid-tight manner to the edges of holes 3' and 3" formedin the sphere 1 at the end of the vertical diameter thereof, so that nodirect communication is provided between the inner space of said sphere1 and that of said duct 3; a bellow 4 is inserted in the return duct 3to absorb the stress developed by the thermal expansion between saidduct 3 and sphere 1. On the other hand, this sphere 1 is surroundedcompletely by an external wall 5, also closed and fluid-tight butconsiderably thinner than the wall of sphere 1; this outer wall mayconsist for example of steel sheets about 0.2" thick; in the form ofembodiment illustrated in FIGURE 1 it consists of two semispherical cupsS and 5" of a diameter slightly greater than that of said sphere 1 andhaving their registering edges assembled in a fluid-tight manner, forexample by welding, to a relatively shallow cylindrical belt member 5"'located in the vicinity of the equatorial plane of the inner sphere 1.The outer wall assembly 5 is also supported by the legs 2 of the tankstructure to which the lower semi-spherical cup 5" is fastened also in afluid-tight manner, notably by welding. The outer face of wall 5 iscovered completely with a layer 6 of asbestos about l" or 1% thick,applied preferably by spraying and covering preferably also the outersurface of each leg 2. Between the sphere 1 constituting the internalwall and the external Iwall S an annular space 7 is formed which willhereinafter be referred to as the annular chamber; the radial dimensionof this annular chamber increases from a minimum value approximating 2"at the bottom of the sphere 1 to a maximum value approximati-ng 6" atthe top thereof, this radial dimension being approximately V4" in theequatorial plane of the sphere (this radial dimension having beenconsiderably exagigerated in FIGURE l in order to make it moreapparent); therefore, the annular charriber 7 has at all points arelatively reduced value in proportion to its diameter whichapproximates that of said sphere 1, and its cross-sectional dimensionincreases gradually from the lower end to the upper end of said chamber,as a consequence of its variation in diameter.

In the form of embodiment illustrated in FIGURE 1 a relatively greatnumber of drain pipes y8, 8 (for example up to six or eight) aredisposed preferably at spaced intervals about the vertical axis of thesphere 1, so as to permit the communication between the base of thissphere and the cylindrical bottom 9 of the external wall 5. rEach drainpipe y8, 8 extends in a fluid-tight manner through the external wall 5and the section thereof located externally of this wall 5 comprises avalve 10 or 10 the opening of which is controlled by a thermometricpickup inconporated in the valve, this pickup consisting for example ofa simple fusible eleme-nt so dimensioned that it will melt ywhen thesurrounding temperature exceeds the preselected dangerous value, thuscausing the automatic opening of the corresponding valve 10 or 10.

The upper portion of the external Wall 5 of the tank opens into the baseof a cylindrical dome 11 having a vertical axis coincident with thevertical axis of the splhere 1, this dome consisting for example andlikewise of relatively thin sheet metal and being assembled in afluidtight manner, for` example by welding, to the top portion of saidwall v5. From the top of this dome 11 emerges a duct 12 leading to oneor a plurality of open-air burners (not shown in FIGURE l).

This open-air burner syste-m is located preferably as remotely aspossible (horizontally and/ or vertically) from the tank. `On the otherhand, the dome 11 contains known means acting as gas-liquid separatorsand in the example illustrated in FIGURE 1 these means consist of afunnelshaped member 13 coaxial with the sphere 1, the lower tubularportion of this funnel opening into the upper orifice 3 of the ret-urntube 3'; of an annular trough-like member 14 secured to the inner sidewall of dome 11 so as to guide the liquid dripping along this walltowards the upper collector of said funnel 13, and, finally, of afiltering mesh structure 1S covering the complete cross-sectional areaof the cylindrical dome 11, in the upper portion thereof.

Conventional Valves are provided in the known fashion to permit thepicking up of liquefied gas from the lower 6 portion of the sphericalcontainer 1 through the external wall 5, but they are not shown inFIGURE l.

The asbestos layer 6 may also be covered in turn with a thin externalshell of polished metal such as aluminium.

When the tank according to this invention is put into service, the aircontained in the annular chamber 7 is forced out and replaced by aninert gas such as nitrogen, retained therein by a slight overpressure(for example 1A p.s.i.) 'by a gaged exhaust member of known type such asa breaking disk 23 mounted notably at the inlet of tube 12.

When the surrounding temperature in the vicinity of lthe tank exceeds apreselected critical Value considered as dangerous for various reasons,such as the breaking out of a fire in component elements of theliquefied gas storage plant Iwhich are located in the vicinity of thetank, the melting of the fusible elements equipping the valves 10, 10etc. causes the nearly instantaneous opening of said valves, whereby theliquefied gas enclosed in the sp'here 1 for example up to the level 16begins to flow through the pipes l8, 8' etc., at the bottom of thesphere 1, into the bottom 9 of the external wall 5 so as to graduallyfill same and rise in the annular chamber 7 where the pressure increasesuntil it breaks the disk 23. Since this annular chamber 7 nowcommunicates with the external atmosphere through the pipe 12 and theopen-air burner system to which this pipe is connected, the liquefiedgas, of which the pressure in the sphere 1 was that of its saturatingvapour, undergoes a considerable pressure reduction; at the same time,the intense heat flux having caused the melting of the fusible elementsassociated with the valves 10, 10 etc. is transmitted to the liquefiedgas, in spite of the asbestos layer `6 covering same, to the externalwall 5. As in the evaporator of a refrigeration system, the liquefiedand expanded gas rising in the annular chamber 7 begins to boil (forexample at about 22 F. (-30 C.) in the case of propane, with dueconsideration for the pressure losses in the annular chamber), so as toabsorb at least one fraction of the flux of external heat transmittedthereto. The liquid and gas mixture attaining lthe upper end of theannular chamber 7 is separated by the above-described means 13, 14, 15mounted within the dome 11, the gaseous fraction being sucked up throughthe duct 12 and directed t0 the open-air burner system where it isburned off at a distance from the tank sufficient to prevent thisfraction from adding extra fuel to the fire to which the tank isexposed, the liquid fraction being returned on the other hand by thefunnel 13 and return duct 3 to the bottom 9 of the external wall 5, Thusthe return duct 3` will produce a continuous circulation of boilingliquefied gas from the lower end to the upper end of the annular chamber7; the boiling liquefied gas circulating therein constitutes a kind ofheat screen about the sphere 1, thus protecting the liquefied gas stillcontained therein against the intense heat flux from the outside; thecomplete tank structure may be so dimensioned that this protection beperfect or, in other words, that even in case of very intense externalheat ux such as caused by a very violent fire in close proximity of thetank the temperature and therefor the pressure of the liquefied gasstill present in the sphere 1, instead of gradually increasing as in aknown tank, will not only remain constant but even decrease below therelatively high lbut not dangerous values which they had attained justbefore the protection measure were started. In this case the tankoperates as a real opencycle refrigeration machine fed from theliquefied-gas tank and taking heat both from the surrounding atmosphereand from the container in which liquefied gas is stored, since theannular chamber 7 is in heat-transfer contact with both media. As longas the fiux of external heat is continued the level 16 of liquefied gasin the spihere 1 decreases and a corresponding quantity of gas is burnedoff in the open-air burner system without allowing in any case thepressure in the sphere 1 to exceed the rated or test pressure lvaluewhich may then not exceed the value contemplated and prescribed byofficial regulations in force. When the external lire is extinguishedthe vaporization of liquefied gas, if any is still present in the sphere1, will firstly decrease and then cease. The tank may possibly stillcontain a considerable fraction of the initial liquefied gas underconditions in which a tank of known type, provided with onlyconventional safety valves, would be completely empty. -If on the otherhand .the duration of the external tire were such that the tank willeventually be fully exhausted, the protection provided 4by the boilingof the liquefied gas will cease at the same time as the risk ofexplosion created by the presence of this liquefied gas in the closedcontainer 1.

The above-described form of embodiment of the tank according to thisinvention is susceptible of many modifications and variations as willreadily occur to anybody conversant with the art, but it is obvious thatthese modiiications and variations would not constitute a departure fromthe basic principles of the invention. Thus, more particularly, theshape and dimensions of the closed tank or container are immaterial, andbesides this invention is also advantageously applicable to existingtanks, irrespective of their shape, for example to cylindrical tanks;this invention is also of considerable interest both in the case ofrelatively small tanks, for example in domestic installations, and inthe case of average-capacity or large-capacity tanks of industrialplants, notably in storage zones of oil refinery plants. Of course, thedimensions of the annular chamber, notably its width, must be adapted tothe dimensions and therefore to the volumetric capacity of the tanksurrounded thereby, in orderA to provide the necessary efficiency andprotection characteristics. In all cases the external wall may consistof a relatively thin material since as a rule there is no difference inpressure between its two faces. As contrasted with the structureillustrated in FIGURE l, the valves provided for connecting the bottomof the closed container to the corresponding end of the annular chambermay be secured directly in or on the wall of the lower portion of saidclosed container, instead of being located externally of the outer wallof the assembly, so as open directly into the annular chamber, forexample above the bottom thereof; of course, in this case thethermometric pickups controlling the opening of said valves must stillbe located externally of the tank so as to be responsive to thetemperature of the surrounding medium; save for this requirement, thesethermometric pick-up devices lend themselves to a great number ofdifferent forms of embodiment, many of which are well known to thoseconversant with the art and are adaptable to the present invention;besides, it may be advantageous to associate with the various valvesthermometric pickups of different designs, although their operation iscontrolled by the same critical temperature; some of these pickups maycomprise for example fusible elements, others may comprise bimetallicstrips, etc., in order to increase the probability of opening at leastone of the valves aforesaid which are relied upon for starting theprotection measures. If the valves are secured in or to the surface ofthe inner wall of the container, the opening of at least some of thesevalves may also advantageously be controlled by the pressure prevailingin the inner space of the container, by means of at least one manometricpickup device disposed in said inner space and possibly incorporated inthe valve; in this case the last-named valve may be in the form of arelief valve opening into the annular chamber, notably a breaking-diskvalve. The dome 11 and the separator means 13, 14, 15 mounted thereinare also adapted to be constructed in many different manners as to theirshapes, dimensions and forms of embodiment which are also well known tothose conversant with vaporization techniques. Finally, the asbestoslayer provided on the outer surface of the external wall of the tankaccording to this invention is optional.

Experience having taught that the damages caused by the bursting of aliquefied-gas tank carried by legs are further increased by the breakingof at least some of these legs and by the fall of the tank, it isadvantageous, in this case, to provide, according to another featurecharacterising this invention, means for deriving into inner ductsformed in these legs one fraction of the boiling liquefied gascirculating in the annular chamber in case of abnormal increase in theexternal temperature; in the tank construction illustrated in FIGURE l,of which one leg is shown in vertical section in FIGURE 3, these meansconsist in providing inside the tubular -legs 2 a cylindrical tube 18having a vertical axis and its upper end supported for example by thewall of the inner sphere 1, at least one aperture 19 being providedbetween its lower end and the base of leg 2; thus, the liquefied gasrising in the lannular chamber 7 penetrates into each tube 18 and thenrises in the annular space formed between this tube 18 and the tubularleg 2, to re-enter the annular chamber 7 through the lateral ports shownat 21. This circulation of boiling liquefied gas (at about 22 F. or 30C. in the case of propane) will cool the leg 2 and concurrently with theexternal asbestos layer 6 protect this leg against the heat flux. Ofcourse, this cooling arrangement may be modified in many differentmanners as will readily occur to those skilled in the art.

The spherical tank illustrated diagrammatically in FIG- URE 4, consistsessentially of an external spherical steel wall 5 constituting a closedand sealed chamber, the thickness of this wall 5 being consistent withthe maximum permissible internal pressure; thus, in the case of apropane tank having a volumetric capacity of 1,000 cubic meters (35,000cu. ft.) and therefore a diameter of about 40 feet, the thickness of theouter wall 5 must be approximately 1.34 (34 mm.) if the maximumpermissible temperature ofthe liquefied gas is 122 F. (50 C.) which, inthe case of propane, corresponds to a maximum permissible internalpressure of the order of 230 p.s.i. This spherical wall 5 opens at itslower end into `a kind of cylindrical vat 9 closed by a bottom 35 whichmay be detachable or not and covered with an external protection layer 6of asbestos; this tank is supported by a plurality of vertical legs 2 of-which only two are shown in FIGURE 4. Internally of the external wall 5an inner wall 1 of relatively thin steel sheet is secured by adequatemeans (not shown), the thickness of this inner wall being preferably ofthe order of 0.08 for example in the case of a tank having the numericalcharacteristics mentioned hereinabove. This inner Wall 1 is so shapedand dimensioned'that when it is secured within the outer wall 5 thedouble-wall structure thus obtained provides therebetween an annularchamber 7 of which the radial width varies preferably from the lower endto the upper end as illustrated in the figure, the annular chamber 7having for example a minimuni radial width in the equatorial plane ofthe sphere 5 and a maximum width at the ends. The thin inner wall 1 isconnected at its lower end to a neck 1 projecting considerably into thevat 9, and its upper end is wide open as at 1 so that the annularchamber 7 of the double-Wall structure will communicate with the innerspace of the tank, that is, the inside of the inner wall 1. Disposednear the upper pole of the spherical external wall 5 are safety valves10, 10 inserted in pipe lines extending through the external wall 5 andleading to a common open-air burner system 12. (not shown). In thisspecific form of embodiment, two groups of safety valves, respectively10 and 10', are provided and have different characteristics andfunctions. The single safety valve 10' of the lirst group has arelatively small cross-sectional passage area and is gaged to open forexample when the liquefied gas contained in the tank has been heated,for example in case of intense sunning, up to a temperature of about 86F. (30 C.); in other words, the safety valve 10 is so gaged that it willopen when the pressure prevailing in the tank is of the order of p.s.i.,which is the vapour tension of propane at 86 F. (30 C). Both valves 10of the second group aforesaid have on the other hand definitely largercross-sectional passage areas; they are gaged to open when the gascontained in the tank has been heated, for example as a consequence of afire in the vicinity of said tank, to a temperature approximating 122 F.(50 C); in other words, the safety valves 10 yare gaged to open at themaximum permissible internal pressure of 230 p.s.i., which is the vapourtension of propane at 50 C. Of course, this second form of embodiment ofthe 'tank according to this invention may be equipped With any desirednumber of groups or series of safety valves, of which the respectivevalves, of which the number is immaterial, are `gaged to open atstepped, predetermined pressure values, their cross-sectional passageareas being also stepped accordin ly.

gAt the upper end of the annular chamber 7, that is, above the upperaperture 1" of the inner wall 1, known separator means such as thoseillustrated in the lower portion of the dome 11 of FIGURE 1, may beprovided; in this case they consist of an annular trough 14 secured tothe inner surface of fthe outer Wall 5, and of a funnel 13 underlyingthe aforesaid annular trough 14 so that its discharge orifice overliesthe upper aperture 1 of the inner Wall 1. Beneath the separator means 13and 14 and above the edge of the upper aperture 1 of said inner wall 1is an annular baflie or defiector 45 also secured to the internalsurface of the external wall 5.

Under normal service conditions the liquefied gas in equilibrium withits vapour at room temperature rises to the same level 16 in the spaceenclosed by the inner wall 1 and in the annular chamber 7. The heatpenetrating into the tank from the outside firstly increases thetemperature of the external wall and is subsequently absorbed by themajor portion of the liquefied gas contained in the annular chamber 7below the level 16. As a result, a certain quantity of the liquefied gascontained in the annular chamber 7 is vaporized, thus tending to raisethe vapour pressure in the upper portion of said annular chamber 7 tothe value corresponding to the thus increased temperature of theliquefied gas contained in its lower portion. As the upper ends of theannular chamber 7 and of the internal wall 1 communicate through therelatively wide aperture 1' and as the gas pressure in said inner wallspace 1 is constant, the gas vaporized in the annular chamber 7 risestowards the top of this chamber and carries along a certain amount ofliquid, so that a propane emulsion will impinge against the annularbafile 45 and be directed thereby towards the upper opening 1" of theinner wall 1. While the largest drops of liquid fall back through thisaperture 1" into the inner space of wall 1, the lighter droplets areentrained by the gas towards the separator means 13, 14; the operationof these separator means (which is already known and therefore needs notbe described in detail) is such that the fine liquid droplets carriedalong by the vaporized gas are returned through the drain orifice offunnel 13 to the inside of wall 1, the practically dry gas accumulatingat the top of the outer wall 5, above the annular trough 14; when thepressure, for instance in the example being described, attains a valueapproximately 140 p.s.i., the safety valve 10 opens and the dry gas isvented to and expands in the atmosphere through said valve and theopen-air burner systern (not shown). This procedure continues as long asheat is transferred from the outside through the outer wall 5, providedthat the temperature of the liquefied gas contained in the tank remainsbelow 122 F. (50 C.) corresponding in this example to the vapour tensionfor which the safety valves 10 are gaged. This process corresponds forexample to the case in which the tank was subjected to intense sunradiation capable to heating its content to a temperature of 85 F. to122 F. (30 to 50 C.). However, if the temperature of the content of thetank according to this specific form of embodiment attained a valueequal to or in excess of 122 F. (50 C.) which is likely to 'be attainedonly in case of a fire breaking out in close vicinity of the tank, theinternal pressure will `reach 230 p.s.i. and the two safety valves 10 ofthe second -group will open in turn, and owing to their'crosssectionalpassage areas considerably greater than that of valve 10', they willpermit the escape of the much greater mass of gas resulting from theconsiderably more intense evaporation caused in the annular chamber 7 bythe heat radiation emitted from the seat of the fire. However, if thetransfer of heat from the outside through the external wall 5 decreasedor were even stopped completely, for example in case of decrease of thesun radiation or if the re were extinguished, the internal pressure willdecrease again, thus causing all the valves previously open to close,inasmuch as, during the above-described procedure, only dry gas haspassed through these valves, due to the provision of separator means 13and 14, and accessorily of baflie 4S, so that these valves have beensafely protected against flooding and possible damages, and also againstinsuficient closing for instance as a consequence of an intensebombardment thereof by a liquid. Thus, any loss of gas ceasesimmediately as the safety valves are reclosed and therefore ready tooperate again in case of a subsequent increment in the externaltemperature.

Like the one of the tank illustrated in FIGURE 1, the external wall 5 ofthe tank illustrated in FIGURE 4 can be covered externally with anasbestos layer applied and secure for examples by spraying. In FIGURE 4,this asbestos layer 6 is provided only on the outer surface ot' theupper portion of the external wall 5 which is normally exposed to themore intense heating due to the fact that it is not cooled lby theevaporation of liquefied gas, notably in case the open-air burner systemthrough which the gas escaping from the safety valves 10 and 10 wererelatively short in proportion to the diameter of the spherical tank.

The cooling of the legs 2 of the tank illustrated in FIG- URE 4 is alsocontemplated by circulating liquefied gas therein, this liquefied gasbeing taken from the annular chamber 7 and directed through the doubletubular path formed in each leg, as illustrated in section in the lowerleft-hand portion of FIGURE 4.

The relatively thin internal wall 1 may consist of sheet metal, forexample .08 thick. Of course, the shape, thickness, dimensions andrelative disposal of the two walls 1 and 5, as well as the number,arrangement and dimensions of the various safety valves 10 and 10', ofthe separator means 13 and 14, and also of baffle member 45 aresusceptible of practical embodiments differing more or less from thatillustrated in FIGURE 4.

The essential component elements of the spherical tank illustrateddiagrammatically in FIGURE 5 are akin to those of the spherical tankshown in FIGURE 4; therefore, the homologue elements of these twostructures are designated by the same reference numerals in these twofigures, in order `to facilitate their identification. However, in themodified form of embodiment illustrated in FIG- URE 5 the lower aperture1 of the thin internal wall 1 has not a neck-like lower extensionopening into a vat forming the extension of the external Wall 5 belowthe tank (elements 9 and 35 of FIGURE 4). On the other hand, the form ofembodiment illustrated in FIGURE 5 comprises in addition a series ofpreferably squareor rectangular-sectioned ducts or pipes 46, 46', asshown in the fragmentary section of FIGURE 7. These ducts extend invertical meridian planes of the concentric spherical walls land 5 so asto engage the inner surface 0f the external wall 5 to which they aresecured preferably by welding, as shown in FIGURE 7. These ducts 46 and46 have preferably a constant cross-sectional passage area throughouttheir length, as contrasted with the horiz-ontal sections of the annularchamber 7 formed by the concentric spherical walls 1 and 5 whichdecrease towards the equatorial plane of these walls, as alreadyexplained hereinabove in connection with the form of embodimentillustrated in FIGURE 5; the internal wall 1 of relatively thin metalsheets consists preferably of separate segments (la, 1b, etc. in FIGURE7) having their vertical edges secured to saidducts 46 and 46 at asuitable distance from the external wall in order to obtain the desiredwidth of the annular chamber 7, whereby each compartment of this chamber7 which is formed between any pair of adjacent ducts 46, 46' constitutesa fiuid-tight enclosure; thus, for instance, the edges 1a, '1b etc. ofthe internal wall 1 are bolted or screwed to lateral angle members rigidin turn with the radial faces of the square-sectioned ducts 46, 46', asshown diagrammatically at 46a and 4611 in FIGURE 7. The lower ends 47,47' of the various, substantially vertical ducts 46, 46 open near thebottom aperture 1' of the internal spherical wall 1, and their upperends 48, 48 open each into a vent or exhaust member 49, 49'communicating `with the atmosphere (FIGURES 5 and 6). In the form ofembodiment illustrated in FIGURES 5 t-o 7, this exhaust or vent memberconsists essentially of a pipe section 49 mounted in a uid-tight mannerthrough t'ne external wall 5 and normally closed by a disk S0 ofsuitable fusible material; a side neck 51 connects this exhaust or ventmember with the surrounding atmosphere either directly or through themedium of the open-air burner system 12 (not shown in FIGURE 5), towhich the safety valves and 10' are also connected as already explainedin detail hereinabove. On the other hand, the wall of each internal duct46, 46' has secured therein, preferably by welding, near its upper end48, a non-return ball-valve 52 normally connecting the upper end 48 ofthe relevant duct 46, 46' with the inner space of the tank, that is, theinterior of its inner wall 1.

If a fire breaks out in close vicinity of the tank illustrated in FIGURE5, and assuming that this tank is filled for example up to the topmostlevel indicated by the dash line 16, the protection of this tank againstany risk of explosion is ensured firstly exactly under the sameconditions as already explained in detail hereinabove. As the liquefiedgas has the same level in the internal space 1, in the annular chamber 7and in the aforesaid ducts 46, 46' having their lower ends 47, 47constantly open, the same pressure prevails in the topmost portion ofthe tank and also in the upper portions of said ducts 46, 46', so thatall the ball valves such as 52 are open as shown in FIGURE 6. As theliquefied gas circulating upwards through the annular chamber 7 ispartially evaporated and subsequently separated and exhausted to theatmosphere in its vaporized state through the safety valves 10, 10',thus reducing the level of liquefied gas in the tank, the activecirculation in said annular chamber 7 decreases gradually; if theliquefied gas in the tank further decreases down to a mean level 16'shown in thick line in FIGURE 5, the active circulation in annularchamber 7 remains sufficient to cause a mixture of liquid and moistvapour to continue to emerge from the upper end to the annular chamber7; it is only if the fire continues, together with the procedure setforth hereinabove, and as the liquid level in the tank drops below themark shown by the dash line 16" in FIGURE 5, which corresponds forexample to a residual level of liquefied gas of about 40" in thespherical tank having a diameter of about 40 feet, and if the safetyvalves 10 and 10 open for example under a rated internal pressure of 230p.s.i. and recloseunder a rated internal pressure of 115 p.s.i., thatvapour beginning to be overheated escapes through the upper end ofannular chamber 7, this vapour being thereafter unable to absorb theheat continuously transferred through the external wall S from the seatof the fire. Of course, the degree of vapour overheat and its inabilityto absorb the heat iiux from the exterior increase at a given momentfrom the lower portions to the upper portions of the annular chamber 7,and at a given point of this annular chamber, as the level of residualliquefied gas decreases in the tank. Under these conditions, thematerial (as a rule steel) constituting the outer wall 5 will nearlyinevitably attain near the top of annular chamber 7 local temperaturesso high that it will not be strong enough to withstand the risinginternal pressure. However, the risks of explosion in connection with apossibly breaking of the external wall 5, at least in the upper portionsthereof, are eliminated by the present invention because the temperatureincrement in the portions of the external wall 5 which are adjacent toeach exhaust member 49, 49' causes the melting of the fusible disk 50closing the upper end of each substantially vertical duct 46, 46 thevacuum or suction resulting therefrom in the upper end 48 of each duct46, 46' will immediately reclose the corresponding ball valve 52; theoverpressure prevailing in the tank will then force out the residualmass of liquefied gas still contained in the bottom of this tank,through the lower ends 47, 47' of the substantially vertical ducts 46,46' in which this still liquefied gas rises so as to be subsequentlyexhausted either directly through the side pipe 51 of exhaust members 49or through the medium of the open-air burner system 12 (not shown), tothe atmosphere where this still liquefied gas is burned off; in casethis residual mass of still liquefied gas were burned in the atmosphere,no additional risk would result for the tank structure, provided thatthis combustion takes place at the end of an open-air burner systemlocated at a relatively great distance from the upper portion of thetank in proportion to the tank diameter. Under these conditions the tankcontains only gas of which the pressure constantly decreases slowly dueto the provision of the exhaust -members 49 left open; any `breaking ofthe external wall S of the tank is thus safely prevented.

The form of embodiment illustrated in FIGURE 5 lends itself to manymodifications and variations of which a great number `will readily occurto those skilled in the art; more particularly, the members 49 foryventing the gaseous products to the atmosphere, which are connected tothe corresponding upper ends of the lvarious vertical ducts 46, 46'- canbe constructed with many modifications and differ considerably from thestructure illustrated in detail in FIGURE 6; thus, one-way valves,non-return valves, relief valves, etc. of very different types, andresponsive to a properly adjusted thermometric pickup, and so disposedas to be sensitive to the temperature of the external wall 5, notably inthe portions most exposed to a thermal overload, that is, in thevicinity of the upper end of the annular chamber 7, may be used. Thevalve 52 is also adapted to be constructed in many different manners.The arrangement of the aforesaid vertical ducts 46, 46' is alsooptional; thus, more particularly, it is not absolutely necessary thatthese ducts be regularly spaced along the circumference of the annularchamber 7; their number and also their shape and crosssectionaldimensions are also optional.

The alternate form of embodiment of the spherical tank according to thisinvention which is illustrated in FIGURE 8 of the attached drawingsdeparts from the structure shown in FIGURE 4 only by the followingpoints (the same reference numerals designating in both figures thehomologue component elements): a cylindrical vat 9, closed by a possiblydetachable bottom 35 coated with an asbestos layer 6', is connecteddirectly with the bottom of the outer wall 5, has an external extensionbeneath the spherical tank proper, supported above the ground by legs 2,of the length relatively considerable in proportion to the diameter ofsaid spherical tank, this length being for example eight feet in thecase of a tank about 40 feet in diameter, the diameter of thiscylindrical extension being of 20" to 28"; similarly the neck 61 wherebythe thin inner wall 1 opens at its lower end into said vat 9 has anextension within the vat of nearly the same vertical length so as toform a chamber in the form of a cylindrical ring 7 of which the upperend communicates directly with the lower end of the annular chamber 7.

FIGURE 8 shows clearly that in case of prolonged fire externally of thetank there eventually remains in the double-walled cylindrical element9, 61 a reduced mass of still liquefied gas, for example 35 cubic feet,whereas the spherical tank proper has been drained nearly completely ofthe liquefied gas previously contained therein, due to the gradualvaporization of this liquefied gas and to its discharge into thesurrounding atmosphere through the safety valves 10" and possibly theopen-air burner system 12 (not shown). On the other hand, the smallresidual mass of still liquefied gas then contained in the lowerdouble-walled cylindrical vat 9, 61 will eventually be vaporizedcompletely; this vaporization taking place rvery rapidly if the fire iscontinued; therefore, any risk of violent explosion is safely avoided incase of a possible perforation of the upper portion of the external wall5 of the tank.

In the form of embodiment illustrated in FIGURE 8, means notably in theform of radial pipes 62, are also provided for diverting towards theinner ducts formed in the legs 2 of the tank structure one fraction ofthe liquefied gas contained in the lower portion of the double-wallcylindrical element 9, 61, in order to cool these legs 2 in case offire, due to a liquefied-gas circulation subsequently returned to theannular chamber 7 at the upper ends of these legs. However, thislast-described pipe arrangement is optional; it is neverthelessadvantageous in that it will further accelerate the completevaporization of the relatively small residual mass of liquefied gasremaining in the cylindrical double-wall element 9, 61, for, asillustrated by the arrows in FIGURE 8, it promotes the circulation inthe lower portion of the annular chamber 7.

In all the above-described forms of embodiment the followingimprovements may further be provided:

Venting members connected for example to the openair burner system aremounted through the external wall, preferably in the upper portionthereof, the opening of these venting members, for example normallyclosed by fusible disks, being controlled by the temperature attained bythe external wall in case of fire.

With this arrangement it is also possible to reduce considerably thepressure prevailing in the tank, notably when the latter has beendrained completely, in case of fire, of its liquefied-gas content, bythe means described hereinabove.

In this case and preferably a gaged valve is mounted in series with each`venting member leading to the atmosphere, this Agaged valve beingadapted to open at an internal pressure lower than the pressurenecessary for reclosing the aforesaid safety valves; this specificarrangement is of course also applicable to the exhaust members 49 ofFIGURES 5 and 6. These two last-named arrangements are such that theypermit of maintaining a residual pressure, in the tank from which allliquefied gas has been drained out, which is only slightly in excess ofthe atmospheric value, and therefore of rendering practically harmless apossible perforation of its external wall weakened by the heating incase the fire continued after the tank has -been drained out completely.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, -as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

What I claim is:

1. A tank for a liquefied gas, comprising an inner wall and a closed,outer wall, supported in a close relationship to each other with anarrow, substantially annular space extending there-betweensubstantially from the lower ends of said walls to the upper ends of thesame, the liquefied gas being contained at least inside of said innerWall, means to connect to each other the respective lower ends of saidannular space and of the inside of said inner wall at least when thetemperature outside of the tank increases dangerously, whereby theliquefied gas rising upwardly through said annular space is partlyvaporized by absorbing the heat transmitted from the outside of thetank, and, near to the upper end of said annular space, means comprisingbafiies to separate the vaporized gas from the still liquefied gas,means to exhaust the vaporized and separated gas in the atmosphere,collector means to collect the separated, still liquefied gas and tubemeans to return the collected, still liquefied gas down to the lower endof said annular space.

2. A tank according to claim 1, in which said gas exhausting means isconnected to an open-air burner system.

3. A tank according to claim 1, comprising further supporting legs,inside of said legs passages for the liquefied gas being cir-culatedtherethrough, and means to connect said inner passages of the legs withsaid annular space.

4. A tank according to claim 1, in which a layer of asbestos is providedon the outer face of at least the upper portion of said outer wall.

5. A tank according to claim 4, in which a thin shell of a polishedmetal is provided on the asbestos layer.

6. A tank for a liquefied gas, comprising a closed inner wall adapted towithstand a predetermined, inner overpressure, and containing theliquefied gas, a substantially thinner, closed, outer wall supportedaround said inner wall in a close relationship thereto with a narrow,substantially annular space extending between said inner and outer wallssubstantially from the lower ends of said walls to the upper ends of thesame, said annular space being normally closed and containing an inertgas, means to connect to each other the respective lower ends of saidannular space yand of the inside of said inner wall only when thetemperature outside of the tank increases dangerously, whereby theliquefied gas rising upwardly through said annular space is at leastpartly vaporized by absorbing the heat transmitted from the outside ofthe tank, and, near to the upper end of' said annular space, means toseparate the vaporized gas from the still liquefied gas, and to freelyexhaust in the atmosphere first the inert gas, then the vaporized andseparated gas, and duct means for returning the still liquefied gas fromthe upper end of said annular space down to the lower end of the same.

7. A tank according to claim `6, in which said return duct extendsvertically through the inside of said inner wall.

8. A tank :according to claim 6, in which said means to connect to eachother the respective lower ends of said annular space and of the insideof said inner Wall consists of at least one valve adapted to said innerwall, and of at least one pickup device for controlling the opening ofsaid valve.

9. A tank according to claim 8, in which the pickup devices aresensitive to the temperature outside of the tank.

10. A tank according to claim 8, in which the pickup devices aresensitive to the pressure inside of the inner wall of the tank.

11. A tank according to -claim 10, in which said connecting meansconsists of at least one safety valve with a breaking disk.

12. A tank according to claim 6, in which said means to connect to eachother the respective lower ends of said annular space and of the insideof said inner wall comprises at least one duct having a sectionextending outside of said outer wall, at least one valve inserted insaid outer duct section, and at least one pickup device sensitive to thetemperature outside of the tank and incorporated to said valve.

13. A tank according to claim 12, in which the valve is normally closedby a fusible element.

14. A tank according to claim 6 in which a dome is mounted externally onthe top portion of said outer wall,

with a free communication 4between the lower end of the inside of saiddome and the upper end of said annular space, said gas from liquidseparating means and means to guide the separated liquid into the returnduct are disposed inside of the lower part of said dome, and said gasexhausting means is arranged at the upper end f said dome.

15. A tank according to claim 6, in which said gas exhausting means isnormally closed by a pressuregaged member.

16. A tank for a liquefied gas, comprising a closed, outer wall adaptedto withstand a predetermined, inner overpressure, a substantiallythinner, inner wall with large openings at its upper and lower ends,said inner wall being supported inside of and in a close relationship tosaid outer wall with a narrow, substantially :annular space extendingbetween said inner and outer Walls substantially from the lower ends ofsaid walls to the upper ends of the same, the respective upper and lowerends of said annular space and of the inside of said inner Wall beingpermanently connected to each other through said large openings in saidinner wall, whereby the liquefied gas normally contained in said annularspace is at least partly vaporized by absorbing the heat transmittedfrom the outside of the tank and lthereby -rises in said annular spaceup to its upper end, and, near to said upper end of the annular space,means to separate the vaporized gas from the still liquefied gas and toreturn said still liquefied gas into the inside of said inner wallthrough its upper opening, and at least one safety valve gaged to openinto the atmosphere at a first, predetermined, inner pressure and tore-close at a second predetermined, inner pressure, lower than saidfirst,- predetermined, inner pressure.

17. A tank according to claim 16, in which the gas from liquidseparating means consists of a funnel disposed above the upper openingin said inner wall and of an annular trough disposed just yabove theupper rim of said funnel.

18. A tank according to claim 16, comprising further an annular bafiiemember disposed just above the rim of the upper opening in said innerwall to direct thereinto the -gas and liquid mixture rising up to theupper end of said annular space.

19. A tank according to claim 16, comprising a plurality of safetyvalves, gaged to open into the atmosphere at predetermined, stepped,inner pressures, and having also likewise stepped, cross-sectionalpassages.

20, A tank according to claim 16, comprising further a set ofsubstantially vertical ducts, disposed on the inner face of said outerwall and extending substantially from the lower end of said annularspace up to the upper portion of said annular space, the lower end ofeach said duct being permanently open, exhaust members t0 theatmosphere, into which the upper ends of said ducts respectively open,[and at least one pickup device, sensitive to the temperature of theupper portion of said outer wall, for controlling the opening of saidexhaust members at a dangerous, predetermined value of said temperature.

21. A tank according to claim 20, in which said exhaust members to the)atmosphere are mounted through said outer wall, and said temperaturepickup devices are fusible elements, normally closing said exhaustmembers.

22. A tank according to claim 20, in which the upper end of each saidduct is further provided with a valve opening into the inside of saidinner wall.

23. A tank according to claim 20, in which said substantially verticalducts have rectangular cross-sections, and said inner wall consists ofseparate sheet panels having their vertical edges tightly secured to theside walls of said ducts.

24. A tank according to claim 16, which further comprises supportinglegs, and in which said inner and outer walls are provided, beneath andoutside of the tank, with vertical, coaxial extensions having muchsmaller horizontal cross-sections than said inner and outer walls, thelower end of said outer wall extension being closed, whereas the lowerend of said inner wall extension opens freely inside of said outer wallextension.

25. A tank for a liquefied gas, comprising a closed outer wall adaptedto withstand a predetermined, inner overpressure, and having a downwardclosed bottom extension of much smaller horizontal cross-section, asubstantially thinner, inner wall having a large opening at its upperend and a downward open bottom extension of still smaller horizontalcross-section, said inner and outer walls being supported one inside ofthe other and in a close relationship to one another, with theirrespective bottom extensions being coaxial, and 1a narrow, substantiallyannular space extending therebetween substantially from the upper, largeopening in said inner wall to the open, lower end of its bottomextension, whereby the liquefied gas normally contained in said annularspace is partly vaporized by absorbing the heat transmitted from theambient atmosphere and thereby rises in said annular space, near to theupper end of said annular space, means to separate the vaporized Igasfrom the still liquefied gas, to collect the separated, still liquefiedgas and to return the same into the inside of said inner wall throughits upper large opening, above said separating means at least one safetyvalve gaged to open into the atmosphere at a first, predetermined, innerpressure and to re-close at a second, predetermined, inner pressure,lower than said first, predetermined, inner pressure, piping meansinserted between the closed, lower end of said outer wall bottomextension and regions of said annular space at a predetermined levelsubstantially above said wall bottom extensions, said piping means beingarranged outside of the bottom extension of said outer wall, to be inthermal exchange with the ambient atmosphere, whereby the liquefied gas,when contained in Said annular space only at a level substantially undersaid predetermined level, is also partly vaporized in said piping meansby absorbing the heat transmitted from the iambient atmosphere, andthereby rises in said piping means, the still liquefied gas being partlyseparated from the vaporized gas at said predetermined level, and beingreturned downward through said annular space into said outer wall bottomextension, whereas the vaporized gas and the still liquefied, notseparated gas rise again in said annular space up to said separatingmeans.

References Cited UNITED STATES PATENTS 1,544,854 7/ 1925 Mueller et al.62-50 1,979,221 10/1934 Dana 62-50 2,190,366 2/1940 Mead 62-50 2,211,0058/1940 Dick 62-50 2,242,108 5/ 1941 Bullowa et al. 6250 2,293,263 8/1942Kornemann et al. 62-50 2,687,618 8/ 1954 Bergstrom 62-50 2,986,891 6/1961 McMahon 62-45 3,087,311 4/1963 Rousseau 62-52 X LLOYD L. KING,Primary Examiner.

U.S. Cl. X.R. 62-50

